The source code of the P5S website contains the following, via Gematsu:
April 25, 2019.
We will show you a new Phantom Thieves that will turn the world’s cognition on its head.
We are energized by voices that seek satisfaction.
Once again, we will take your heart at the Ryougoku Kokugikan.
—From the Phantom Thieves.
“P5S” is one of the Persona 5 related domains that were registered in December 2018, along with “P5M” and “P5B.”
The “Persona Super Live: P-Sound Street 2019: Welcome to No. Q Theater” concert will take place on April 24-25, 2019. P5R and P5S are two projects which will be officially revealed during the event.